1. Field of the Invention
The present invention relates to a data converter and a method thereof, a data transmission/reception apparatus and a method thereof, and a network system, for example in a network environment comprising a server, a relay node and a client.
2. Description of the Related Art
In recent years, a large number of network applications for transmitting multimedia information such as audio, moving picture, still images, and the like have been provided. A network system capable of transmitting various data has been constructed.
In this kind of network system, the communication band must be wide when transmitting moving picture data of high image quality or the like. Various information compression techniques such as JPEG (Joint Photographic coding Experts Group), MPEG (Moving Picture Experts Group), ATRAC (Adaptive Transform Acoustic Coding), and the like are required as technical elements.
In many cases, however, compression formats such as MPEG and the like are respectively peculiar to the formats and are therefore not compatible with each other. Particularly, when transmitting various multimedia information items in the network system, the encoder in the data transmitter side and the decoder in the data receiver side must have compatibility with one same compression format. In the present situation, a personal computer is often used as a receiver terminal in the data receiver side. Therefore, many kinds of compression formats are covered by installing a plurality of decoder software applications corresponding to the compression formats.
However, in the network system, there are several dedicated terminals that cannot be compatible with a given data format used in a data transmitter side. For example, it is known that a cellular phone terminal performs compression according to a system in which prepared voice vectors (several ten sets of amplitude data) such as CELP (Codebook Excited Linear Prediction) and the like are combined with each other to achieve encoding at a transmission speed of about 10 k bit/sec. The cellular phone terminal, however, can communicate with a normal analogue telephone which does not adopts this kind of method.
As shown in FIG. 1, an audio signal encoded according to the CELP system by a cellular telephone 100 is converted into an analogue audio signal by a CELP decoder 102 through a cellular telephone network 101, and the analogue audio signal is then transmitted to an analogue telephone 104 through an analogue telephone network 103. Thus, in a conventional network system, matching between different formats is achieved by using a dedicated converter which is previously installed, and connection is made between the cellular telephone 100 and the analogue telephone 104.
Meanwhile, in a network system using Internet protocol, information items in complicated various data formats in the transmitter side are transmitted and received, unlike the telephone network using a simple data format in the transmitter side. In addition, one same data transmission apparatus is frequently accessed from receiver terminals compatible with many different data formats.
For example, if a server apparatus 201 stores moving picture data according to MPEG 2 standard and if moving picture data according to MPEG2 is transmitted to a PC 202 as a receiver terminal, as shown in FIG. 2,data is transmitted through a communication route R101 using network nodes 203a, 203c, and 203d as relay nodes. In case where data is transmitted to a PDA (Personal Digital Assistant) 204 compatible with only MPEG4, format conversion is carried out from MPEG 2 to MPEG4 by a format converter 205. Thereafter, moving picture data is transmitted to the PDA 204 through a communication route R102, using the network nodes 203a and 203c as relay nodes. This format converter 205 is supplied by the server apparatus 201 and has a function to convert data from MPEG2 format into any of various data formats in the receiver side.
However, the compression algorithm advances year by year as the algorithm has advanced from MPEG2 to MPEG4. It is known that a system for transmitting data with high quality at a higher compression rate has been proposed. Although a receiver terminal which has a margin for processing ability like the PC 202 can have a plurality of decoder software applications, format conversion should be carried out, as a service of the server apparatus 201 as a data transmitter side, by the format converter 205 in the PDA 204 as a portable device which must minimize power consumption or a terminal which cannot update its own functions.
In addition, the cost for providing services will increase and burden the data transmitter side if the data transmitter side sets format converters 205 each compatible with all formats, including data of formats which are year by year added and stored into the server apparatus 201 in the data transmitter side. In particular, it is often irrational to set such format converters 205 that perform conversion into a data format used less frequently, in all data transmitters.
Further, when transmitting multimedia information, multi-cast communication is used for casting simultaneously information having same contents to a plurality of terminals, as used in current broadcasting. In the multi-cast communication, only one single moving picture data item needs to be transmitted to branched network nodes. However, when terminals at destinations use different data formats, it is necessary to transmit data including same contents in different data formats from the data transmitter side. For example, in FIG. 2, moving picture data having same contents are multi-cast to the PC 202 and PDA 204, two kinds of data which are respectively of the MPEG2 format and the MPEG4 format must be transmitted, where viewed from the data transmitter side. It thus cannot be said that network resources are used efficiently. Further, this problem becomes more conspicuous when it is necessary to multi-cast data in many data formats such as MPEG2, MPEG1, DV, motion JPEG, and the like.